This invention relates to coils for a magnetic levitation apparatus which supports a moving body without contact by utilizing an electromagnetic force produced by electromagnetic induction. More particularly, the invention relates to induction coils in a magnetic levitation apparatus capable of supporting a moving body without contact even if the traveling velocity of the moving body is low.
A magnetic levitation apparatus for supporting a moving body contactlessly by utilizing electromagnetic force produced by electromagnetic induction finds use in electromagnetically levitated railways and electromagnetically levitated conveyance systems. An electromagnetically levitated railway will be discussed as one example of use of an electromagnetic levitation apparatus.
First, reference directions and directions of force will be clarified. The direction in which a moving body travels shall be referred to as the direction of travel. Among the directions lying at right angles to the direction of travel, the direction along which the force of gravity acts shall be referred to as the vertical direction, and directions other than the vertical direction shall be referred to collectively as the lateral direction. The direction of rotation about the traveling direction shall be referred to as the rolling direction. Further, with regard to coil dimensions, the dimension in the vertical direction shall be referred to as coil height and the dimension in the direction of travel shall be referred to as coil length.
In a magnetically levitated railway, a truck serving as a moving body is provided with superconducting coils functioning as magnetic-field generating sources (the coils shall be referred to as a "field generating coils" below), and the side walls of the guideway are provided with short-circuit coils (referred to as "induction coils" below) for generating induced current. If a relative velocity develops between the field generating coils and the induction coils, an induced current will flow through the induction coils in accordance with Lenz's law. Electromagnetic force is generated between the induced current and the field generating coils, thereby making it possible to support the moving body without contacting it.
The advantages of supporting a moving body contactlessly by means of electromagnetic force may be summarized as follows:
(1) High speed is possible.
(2) Load acts upon the guideway while being dispersed. This makes it possible to design the railway less stringently in terms of required strength.
(3) Since there are no points of frictional contact between the moving body and the guideway, the moving body and the guideway are easier to maintain.
(4) The system is environmentally friendly since there is no noise or vibration that ordinarily accompanies contact.
However, the electromagnetic force for supporting the moving body without contact starts to be produced only after relative velocity develops between the field generating coils and the induction coils. The electromagnetic force is not very large when the relative velocity is low.
Further, in order to support the moving body without contact, an electromagnetic force for supporting the weight of the moving body is necessary and an electromagnetic stiffness is required to restore the moving body to a normal position by overcoming external disturbances in the bilateral direction.
This means that it is necessary for the moving body to run on auxiliary wheels in a region of velocities within which the electromagnetic force or electromagnetic stiffness necessary for supporting the moving body without contact is not attained.
When the moving body runs on its auxiliary wheels, it is required that the guideway has tracks for the auxiliary wheels. As a result, this track must be stringently designed for sufficient strength owing to the concentration of load. In addition, since there are points of friction between the auxiliary wheels and the tracks, maintaining the auxiliary wheels and the tracks is not easy.
Accordingly, there is demand for a magnetic levitation apparatus capable of supporting a moving body contactlessly starting from as low a velocity as possible.
The foregoing holds true also with regard to a magnetically levitated conveyance system, in which the only change is that the moving body is a conveyed body rather than a truck.
Reference will now be had to the drawings to describe a magnetically levitated railway as one example of use of a magnetic levitation apparatus according to the prior art.
FIG. 1 is an explanatory view illustrating the manner in which field generating coils and induction coils are connected in a conventional magnetic levitation apparatus, and FIG. 2 is an explanatory view of a magnetic levitation apparatus in the prior art.
Shown in FIGS. 1 and 2 are field generating coils 1, 1', induction coils 2, 2', upper coils 3, 3' of the induction coils 2, 2', lower coils 4, 4' of the induction coils 2, 2', null-flux wires 5, a moving body (truck) 6, a guideway 7, vertical centers 8, 8' of the field generating coils 1, 1', vertical centers 9, 9' of the induction coils 2, 2', a lateral center 10 of the moving body 6, a lateral center 11 of the guideway 7, distances 12, 12' between the vertical centers of the field generating coils 1, 1' and the vertical centers of the induction coils 2, 2', lateral spacings 13, 13' between the field generating coils 1, 1' and the induction coils 2, 2', support wheels 14, 14', and guidance wheels 15, 15'. Further, a, a', b, b', c, c' and d, d' represent the vertices of the upper coils 3, 3' of the induction coils 2, 2', e, e', f, f', g, g' and h, h' represent the vertices of the lower coils 4, 4' of the induction coils 2, 2', and i, i', j, j', k, k' and l, l' represent the vertices-of the field generating coils 1, 1'.
The field generating coils 1, 1' are secured to respective ones of both sides of the moving body (truck) 6. The induction coils 2, 2' through which an induced current flows are secured to the side walls of the U-shaped guideway 7 so as to face the field generating coils 1, 1', respectively, and are disposed continuously in the direction of travel across the entirety of the guideway. The moving body 6 has the support wheels 14, 14', which are for supporting the moving body 6 in the low-velocity region, and the guidance wheels 15, 15'.
The induction coils 2, 2' comprise the upper coils 3, 3' of the induction coils and the lower coils 4, 4' of the induction coils, these coils having the same dimensions and numbers of turns and being arranged in two stages, one above the other. The upper coils 3, 3' of the induction coils and the lower coils 4, 4' of the induction coils are connected so as to be oppositely oriented from each other with respect to the magnetic fields produced by the field generating coils 1, 1'. Further, the upper coils 3, 3' of the induction coils and the lower coils 4, 4' of the induction coils are connected, on the left and right sides of the U-shaped guideway 7, by the null-flux wires 5 so to be oppositely oriented with respect to the magnetic fields produced by the field generating coils 1, 1'.
In a case where the vertical centers 8, 8' of the field generating coils and the vertical centers 9, 9' of the induction coils coincide, the magnetic fields interlinking the upper and lower coils of the induction coils 2, 2' from the field generating coils 1, 1' become zero, no induced current flows and no levitation force is produced.
When the field generating coils 1, 1' descend under the weight of the moving body 6 so that the vertical centers 8, 8' of the field generating coils assume positions slightly below the vertical centers 9, 9' of the induction coils, interlinking magnetic fields develop between the upper and lower coils of the induction coils 2, 2'. As a result, induced current flows and a levitation force is produced. Consequently, the moving body 6 is supported contactlessly at a vertical position at which the weight of the moving body and the levitation force balance. A feature of the levitation force is that the levitation force increases as the distances 12, 12' between the vertical centers 8, 8' of the field generating coils and the vertical centers 9, 9' of the induction coils increase and as the lateral spacings 13, 13' between the field generating coils and the induction coils decrease.
Further, in a case where the lateral center 10 of the moving body and the lateral center 11 of the U-shaped guideway 7 coincide so that the lateral spacings 13, 13' between the field generating coils and the induction coils become equal at each of the left and right coils, the magnetic fields interlinking the left and right coils of the induction coils 2, 2' from the field generating coils 1, 1' become zero, no induced current flows and no force (referred to as a guidance force below) that attempts to move the moving body 6 in the lateral direction is produced. In a case where the vehicle body 6 is displaced in the lateral direction by cross wind or by centrifugal force that is produced when the moving body travels along a curved section of the railway, as a result of which the lateral spacings 13, 13' between the field generating coils and the induction coils differ at each of the left and right coils, interlinking magnetic fields develop between the left and right coils of the induction coils 2, 2'. As a result, induced current flows and a guidance force that attempts to restore the moving body 6 to its original position is produced.
If the moving body 6 is displaced laterally (here a case in which the moving body 6 is displaced in the direction of the induction coil 2 will be taken as an example), the induction coil 2 on the side on which the literal spacing 13 between the field generating coils and the induction coils has narrowed produces a levitation force greater than that produced by the induction coil 2' on the side on which the lateral spacing 13' between the field generating coils and the induction coils has widened. As a result, a counter-clockwise moment is produced in the rolling direction.
The following equivalent guidance stiffness F y y' is used as a guidance stiffness in a case where a large moment is produced in the rolling direction at the time of lateral displacement: EQU F y y'=F y y-M.phi.y.times.F y.phi./ M.phi..phi.
where F y y represents guidance stiffness [(guidance force) (lateral displacement) at time of lateral displacement]; M.phi. y, F y.phi. the coupling stiffness between guidance and rolling [(rolling moment)/(lateral displacement) at the time of lateral displacement, (guidance force)/(rolling angle) at the time of rolling displacement, these two constants generally agreeing with each other]; and M.phi..phi. the rolling stiffness [(rolling moment)/(rolling angle) at time of rolling displacement].
According to the above equation, the equivalent guidance stiffness decreases when the coupling stiffness between guidance and rolling increases. Therefore, the equivalent guidance stiffness in this method becomes small.
When the manner in which each side of the induction coils 2, 2' produces electromagnetic force is investigated, it is seen that levitation force is produced mainly by the central sides cd, c'd', ef, e'f' and that guidance force is produced by the lowermost sides gh, g'h'.
On the other hand, the uppermost sides ab, a'b' do not produce guidance force very much. Since levitation force is produced by the means that the vertical centers 8, 8' of the field generating coils fall below the vertical centers 9, 9' of the induction coils, as the result, the distance between the uppermost sides ab, a'b' and the upper sides ij, i'j' of the field generating coils 1, 1' becomes larger than the distance between the lowermost sides gh, g'h' and the lower saides kl, k'l' of the field generating coils 1, 1'. For this reason, not much guidance force is produced.
Thus, the equivalent guidance stiffness in the conventional method is inadequate in the low-velocity region. In the case of a magnetically levitated railway, it is believed that the velocity at which contactless support starts is on the order of 150 km/h.
The foregoing holds true also with regard to a magnetically levitated conveyance system, in which the only change is that the moving body is a conveyed body rather than a truck.
Thus, in accordance with the prior art, an electromagnetic force for supporting a moving body without contact is not obtained at low relative velocity and neither is electromagnetic stiffness for restoring the moving body to the normal position when the moving body is displaced by external distrubance in the lateral direction.